1. Field of the Invention
The present invention relates to instrumented coupling devices and methods and, more specifically, to apparatus and methods for measuring or predicting bending in a pin member used to couple a joint.
2. Description of the Related Art
A common means of joining two mechanical bodies together involves the use of one or more pin members disposed through aligned and mating apertures. A “pin member” as the term is used herein may be interpreted broadly to include such devices as bolts, screws, pins, axles, and the like. The joint may take any one of a number of forms. It may, for example, comprise a flanged joint between two pipes, pressure vessel housings, a flanged joint between two solid rocket motor casings, clevis and yoke assemblies, etc.
Under operating conditions, a variety of forces act on the pin member. These forces include tension and compression forces along the longitudinal axis of the pin member, or with force components along that axis, shear forces normal to the longitudinal axis of the pin member, torsional forces, and bending moments.
Apparatus and methods have been known for instrumenting a pin member to measure tension, compression shear, and/or torsion. For example, the use of various types of strain gages to measure such forces are well known. Sensing devices also are known in which such strain gages are used in combination with bridge circuitry such as a Wheatstone bridge.
The effective measurement of bending forces, however, has eluded effective measurement. Bending forces in many mechanical systems are particularly incideous forces, e.g., because they typically isolate a relatively large stress in a localized area. It is often highly desirable to design a component or system so that bending forces and their detrimental effects can be minimized. The direction of bending can be important in some applications, for example, because pin member through-hole radial translation with respect to the pin member shaft often can result in unobvious bending directions with respect to the ideal. The measurement of bending forces or moments often is particularly difficult, however, because the direction of the force can change, sometimes rapidly, over time. In the case of solid rocket motor joints, for example, when the rocket motor is fired, bending moments can be vary substantial and their direction can vary widely.